The invention relates to a method of producing Lyocell fibres, in which a cellulose is introduced with a predetermined degree of polymerisation, and from the cellulose, with the addition of a treatment medium, a cellulose solution or initially a cellulose suspension and from this then the cellulose solution is produced, and in which the cellulose solution is extruded to form endless moulded bodies.
The invention also relates to a device for the production of Lyocell fibres with a mixing device to which a cellulose can be fed and in which a cellulose solution, with the addition of a treatment medium, directly or with the formation of a cellulose suspension can be processed, with a spinning head, through which the cellulose solution can be extruded to form endless moulded bodies, and with a conveying device, through which the cellulose suspension and/or the cellulose solution can be conveyed from the mixing device to the spinning head.
These types of methods and devices are known from the Lyocell technology. With the Lyocell technology threads, fibres, films and membranes are extruded as endless moulded bodies from the spinning mass containing cellulose, water and tertiary amine oxide. Due to its environmental compatibility, the Lyocell technology is increasingly replacing the conventional viscose methods. The environmental compatibility of the Lyocell method is based on the solution of the cellulose without derivatisation in an organic, aqueous solvent. From this cellulose solution endless moulded bodies are extruded, for example fibres and films. Through the extrusion of the moulded bodies and the orientation and regeneration of the cellulose in the course of the extrusion moulded bodies of high strength are obtained with versatile possible uses in the textile and non-textile sector. The name Lyocell was issued by the BISFA (International Bureau for the Standardisation of Man-made Fibres). In the state of the art, the Lyocell method is now well documented.
Tertiary amine oxides are known as solvents for cellulose from U.S. Pat. No. 2,179,181 which can dissolve cellulose without derivatisation. From these solutions, the cellulose moulded bodies can be obtained by precipitation.
The processing of the cellulose, dissolved in an aqueous amine oxide, particularly N-methylmorpholine-N-oxide (NMMNO), is however problematical with regard to safety, because the degree of polymerisation of the cellulose on dissolving the cellulose in NMMNO decreases. In addition, amine oxides generally exhibit only limited thermal stability, particularly in the system NMMNO/cellulose/water, and have a tendency to spontaneous exothermic reaction. To overcome these problems and to be able to manufacture Lyocell fibres economically, there is a series of methods for solution in the state of the art.
In U.S. Pat. No. 4,144,080, it is stated that at high temperatures the cellulose dissolves more quickly in a tertiary amine-N-oxide and forms a more homogeneous solution if the cellulose together with the preferred ingredients of tertiary amine-N-oxide and water is milled. In WO-94/28219, a method for the production of a cellulose solution is described in which milled cellulose and an amine oxide solution are placed in a horizontal, cylindrical mixing chamber. The mixing chamber exhibits axially spaced stirring elements that are rotating around its longitudinal axis. Apart from NMMNO, N-methylpiperidine-N-oxide, N-methylpyrolidone oxide, dimethylcyclohexylamine oxide and others can be used as the amine oxide. Mixing in the mixing chamber occurs between 65° C. and 85° C. According to WO-A-98/005702, the cellulose is mixed in a device with the aqueous solution of the tertiary amine oxide, whereby the mixing device exhibits a mixing tool and a container which rotates during mixing.
In WO-A-98/005702, the mixing tool is improved such that it is formed as a paddle, rail or helix and during mixing preferably prevents the formation of deposits on the inner surface of the container. In WO-A-96/33934, a buffer device is described, which comprises a mixing vessel and a conveying screw as a discharging device. In this way, a continuous production of the cellulose solution is facilitated despite the cellulose being fed in batches.
The method of WO-A-96/33934 has been further developed by the method of WO-96/33221, in which a homogeneous cellulose suspension is produced from pulverised cellulose and an aqueous amine oxide solution in one single step. For this purpose, the pulverised cellulose is brought into contact with the liquid, aqueous tertiary amine oxide and a first mixture is formed in this way. The first mixture is spread in layers on a surface and transported under intensive mixing over this surface. This process can be carried out continuously. Other methods in which the cellulose solution is treated in the form of a thin layer are also known from EP-A-0356419, DE-A-2011493 and WO-A-94/06530.
Also the pulverisation of the cellulose itself is an object of patent publications. For example, U.S. Pat. No. 4,416,698 mentions it as an advantage if the cellulose is milled to a particle size of less than 0.5 mm. In WO-A-95/11261, prepulverised cellulose is introduced into an aqueous solution of a tertiary amine oxide to produce a first suspension. This first suspension is then milled and converted into a formable cellulose solution with the application of heat and a reduced pressure. In order to feed back the dust arising from milling or pulverising the cellulose into the process, filters are used in WO-A-94/28215 through which the cellulose dust is separated from the air. In WO-A-96/38625 a system is described which can pulverise both cellulose bales as well as cellulose in leaf form. For this purpose, an ejection hopper is provided which opens into a device for prepulverising the cellulose.
In EP-B-0818469, it is suggested that cellulose is dispersed in aqueous amine oxide solutions and the dispersion thus obtained treated with xylanases.
Apart from these efforts to economically produce a homogeneous cellulose solution capable of being spun, there are also attempts to overcome the problem of the decomposition phenomena of the cellulose solution which occur spontaneously under an exothermic reaction. In Buijtenhuis et al., The Degradation and Stabilisation of Cellulose Dissolved in NMMNO, in: Papier 40 (1986) 12, 615-618, investigation results are described, according to which metals appear to reduce the decomposition temperatures of the NMMNO in the cellulose solution. Primarily, iron and copper appear to speed up the decomposition of NMMNO. Other metals, such as for example nickel or chrome, also exert a negative influence on the decomposition properties of the cellulose solution in appropriate occurrence and appropriate concentration, if they are present in appropriate concentrations. However, in WO-A-94/28210 stainless steel is still used as the material for a spinning head in order to withstand the high pressures during the extrusion of the cellulose solution.
In addition, the system NMMNO/cellulose/water in the highly concentrated NMMNO region has the property of releasing metal ions from the process apparatus, such as lines, filters, and pumps, which reduces the system stability. In WO-A-96/27035, a method for the production of cellulose moulded bodies is described in which at least some of the materials in contact with the cellulose solution contain at least 90% of an element from the group of titanium, zirconium, chrome and nickel down to a depth of at least 0.5 μm. The important aspect with regard to WO-A-96/27035 is that the rest of the composition of the apparatus and piping, where it comes into contact with the cellulose solution, does not contain any copper, molybdenum, tungsten or cobalt. According to WO-A-96/27035, this measure should prevent exothermic decomposition reactions.
Finally, in DE-C-198 37 210, which is taken as the closest state of the art, a homogeneous cellulose solution is produced irrespective of the water content of the cellulose used. In contrast to current methods, here the cellulose is first transported through an initial shear zone in the absence of NMMNO under homogenisation in a pulper and is only then added to a low water-content NMMNO.
Another way of producing the cellulose solution is followed in DE-A-44 39 149 which forms the closest state of the art. According to the method of DE-A-44 39 149, the cellulose is pretreated enzymatically. To increase the effectiveness of the enzymatic pretreatment, the cellulose can be disintegrated before the pretreatment under shearing in water. Then, the pretreated cellulose is separated from the liquor and the separated cellulose is introduced into a melt of NMMNO and water. Hereby, the separated liquor can practicably be fed back to the pretreatment after supplementing the water and enzyme losses. However, in practice, this type of process management has proven to be impracticable, because the cellulose solution obtained in this way is unstable.
Despite these various approaches to obtaining a homogeneous and stable cellulose solution and to convey it through to the extrusion openings, while avoiding exothermic decomposition reactions, the environmentally friendly and economical production of a homogeneous cellulose solution and its stability remain problematical. Furthermore, it is problematical that the cellulose solution ages, which is expressed in an increasing reduction in the degree of polymerisation with time. With some celluloses which are supplied with already a low degree of polymerisation and are processed to form a spinning mass, ageing may lead to unacceptable reductions in quality.